Activated sludge process and system

ABSTRACT

An activated sludge sewage treatment process and system: the process and system include an enclosed aeration chamber providing a vessel for a mixed liquor formed from a liquid biologically degradable waste and recirculated sludge, with an upper region in the enclosed chamber into which droplets of the mixed liquor are generated and a gas containing a major portion of oxygen is injected under pressure. The gas in the upper region is maintained under pressure and oxygen is transferred to the droplets and mixed liquor at an improved rate.

United States atent 1191 1111 3,724,667 McKinney [4 Ar. 3, 1973 s41ACTIVATED SLUDGE PROCESS AND 5,341,150 9/1967 Ciabattari et al... ..210/x SYSTEM 3,219,575 11/1965 Chapman eta] 3,369,343 2/1968 Robb ..55/68 XInventor: Ross E. McKinney, Lawrence, Kans.

[73] Assignee: Air Products & Chemicals, Inc., Ala y Exam e Michaelogers lentown, Pa. Attorney-Ronald B. Sherer [22] Filed: Oct. 28, 1971ABSTRACT [21] Appl' 192L604 An activated sludge sewage treatment processand Related U.S. Application Data system: the process and system includean enclosed aeration chamber providing a vessel for a mixed liquor [63]ggsa of 4227lmne 1970 aban' formed from a liquid biologically degradablewaste and recirculated sludge, with an upper region in the 521 user...210/195, 210/7, 210/14 W chamber which dmplets the mixed 1 210/197,210/219 261/91 liquor are generated and a gas containing a major por-[51] Int. Cl ..C02c 1/10 tion of Oxygen is injected under Pressura Thegas in 58 Field of Search ..210/3-9, 14, 15, the pp region is maintainedunder Pressure and 210/195, 194, 220, 221 ygen is transferred to thedroplets and mixed liquor at an improved rate. 56 R i l e erences cued 5Claims, 1 Drawing Figure UNITED STATES PATENTS 3,547,813 12/1970Robinson et al ..210/15 ACTIVATED SLUDGE PROCESS AND SYSTEM Thisapplication is a continuation of Ser. No. 42,271, filed June 1, 1970,now abandoned.

BACKGROUND OF THE INVENTION This invention relates to an activatedsludge sewage treatment process and system and, more particularly, to aprocess and system which utilizes an enclosed aeration chamber intowhich is injected an oxygen enriched gas which contacts the mixed liquorbeing aerated.

It is well established that domestic sewage, as well as waste fromcommercial and chemical operations which create a biologicallydegradable material, must be treated before being discharged into areceiving body or stream of water. Various processes and systems havebeen proposed for treating such waste. One of the most popular processesis the activated sludge process.

In the activated sludge process, the waste to be treated is aerated inthe presence of bacteria until the bacteria have stabilized the organicmatter in the waste. The bacteria, while stabilizing the organic matter,require a supply of dissolved oxygen in order to function in an. aerobicstate. In addition to the bacteria, which are microscopic plants,microscopic animals, particularly protozoa, must be present in thesystem to remove excess bacteria from the sludge.

The ecology of the system may be traced from the organic nutrients ofthe waste which is fed into the system and serves as food for thebacteria and to the protozoa which live off the bacteria which utilizethe organic matter. Both the bacteria and the protozoa or othermicroscopic animals require oxygen so that the system will be in anaerobic condition and function efficiently, not generating odors andcreating other unwanted effects which result from an anaerobic system.

With activated sludge it is necessary to aerate the sludge so thatoxygen may be transferred from the air to the liquid in the sludge. Thisdissolved oxygen serves as a supply for the bacteria and the protozoa.

In the normal operations, most waste contains sufficient microorganismsto produce an activated sludge without seeding bacteria. However, in anactivated sludge treatment process or system, the time to produce sludgewithout seeding would be too long and uneconomical to be usable.Accordingly, in an activated sludge system, sludge is removed from asettling chamber and recirculated and aerated with the waste being addedso thatthe recirculated sludge supplies the microorganisms necessary tostabilize the material.

In a conventional activated sludge system the waste material which is tobe treated is usually first passed into a primary sedimentation tank orbasin where the settleable solids are permitted to settle out. Theremaining liquor is then transferred to an activated sludge aerationtank or chamber. If desired, the primary sedimentation tank may beeliminated although it is generally good practice in at least largeinstallations to use a primary tank. The effluent from the primary tankis then fed to the activated sludge aeration chamber where the wasteliquid is mixed with sludge containing the necessary microorganisms andwhich has resulted from earlier treated waste. This forms a mixed liquorwhich is aerated fora predetermined period of time in the aeration tank.

The aeration is usually accomplished by means of diffusing air into themixed liquor so that it bubbles upwardly through the liquor or bymechanically stirring or agitating the liquor within the tank so as tocontact the oxygen in the air. These aeration methods are directedtowards increasing the amount of dissolved oxygen in the liquor.

The amount of oxygen in the liquor is of importance since oxygen is oneof the primary limiting factors influencing the efficiency of aconventional activated sludge system. The demand of oxygen is a directfunction of the biological metabolism in the system. The greatest demandfor oxygen may be expected to occur at that portion of the aeration tankor chamber when the food (the form of waste) and the microorganisms aremixed together. If the demand for oxygen is substantially greater thanthe supply, anerobic conditions will set in and problems will develop inoperating the system.

Low dissolved oxygen concentrations result in a turbid effluent from theaeration chamber since the protozoa do not develop to their fullestextent and the desired ecology of the system is disturbed.

In order for an activated sludge process to function properly it isgenerally accepted that a dissolved oxygen residual of at least 0.5 mg.per liter should be present at all times. As pointed out heretofore,oxygen is usually supplied to activated sludge' by either mechanical ordiffused aeration methods. With all prior art diffused aeration methods,as far as it is known, there has been a low rate of oxygen transferefficiency and this has been considered one of the major engineeringproblems with activated sludge systems.

In diffused aeration the oxygen in the air must be transferred from abubble of gas to the liquid and then to the microorganism. Since oxygenis a fairly insoluble gas, there is an obvious resultant low efficiencyof transfer. The rate of oxygen transfer is a function of the oxygengradient existing between the gas and the liquid, the surface area ofcontact between the liquid and the gas, the time of contact,temperature, and the characteristics of the liquid. Adding to theproblems of oxygen transfer when air is used is the fact that onlyapproximately 21 percent of air is oxygen. Accordingly, the contactbetween the oxygen in the air and the liquid is further reduced.

In order to overcome this shortcoming of the precentage of oxygen inair, it has been suggested that substantially pure oxygen be utilizedand diffused in an activated sludge system rather than air.

Generally speaking, with diffusion methods, small bubbles of air oroxygen transfer more oxygen than large bubbles. However, while thebubbles of gas may be small when generated they have a tendency tocoalesce as they rise through the liquid. Therefore, the largerresulting bubbles are spaced apart greater distances than would be thecase with smaller bubbles. As the large bubbles rise in the liquid, theregion about it forms a cloud of oxygen saturated liquid which iscarried along by the bubble. As a result, the oxygen trapped in thebubble cannot be transferred to the liquid beyond the cloud.

With the mechanical aeration methods the surface is agitated or dropletsof liquor are showered down over the tank surface permitting theagitated surface or droplets to be exposed to the oxygen in the air. Thetransfer of oxygen to droplets is more rapid than the transfer of oxygenfrom bubbles of air, because the interfacial film is about as third asthick with droplets. However, it is obvious that the attainable time ofexposure of droplets to air in an open tank or chamber is usually quiteshort. For this reason, in ordinary activated sludge systems, it wouldbe uneconomical to use an oxygen rich gas in place of air since theoverwhelming portion of it would be wasted if one tried to create anoxygen rich atmosphere in the region above the liquor in an aerationchamber and to shower droplets through it.

In addition to the problem of oxygen transfer, one of the shortcomingsof the conventional activated sludge process is its lack of stability.While theoretically simple, the activated sludge process is difficult tooperate, particularly in those cases where the sewage flow or organicloading of the influent varies. Of course, if the load or flow can beretained and fed into the system at a uniform rate, then operationsbecome more constant and more susceptible to control. However, in thosesituations where the load varies, operating problems are very likely tobe encountered.

Generally when microorganisms reach the end of the aeration period, theyare in equilibrium with the organic matter surrounding them. The numberof living organisms at that time is directly proportional to the organicmatter available at this point and a balance is maintained between thebacteria and the microscopic animals. In a normal cycle of domesticsewage treatment, for example, the flow and the organic strength arevery low at night and increase during the day. This means that themicroorganisms will increase during the period of heavy flow or foodsupply and then as the availability of nutrient organic matter isreduced, the

microbial mass will also decrease along with the rate of metabolism. Ifthis occurs, the microorganisms may be lost in the effluent from theaeration tank or chamber. When a new load is introduced the microbialmass must be built up again. This imbalance and oscillation has causedsubstantial trouble in the operation of activated sludge systems. Toovercome this, it has been proposed to provide a relatively constantsystem with a proper balance between the available food or nutrientsupply and the microorganism population. To do this a variation on theactivated sludge system called the complete mixing or completely mixedactivated sludge process has been devised. This system is rapidlybecoming very popular and is usable in plants of varying size from thesmall package plants used for small industrial wastes to disposalsystems for large municipalities.

In the complete mixing activated sludge system, the influent waste ismixed with the mixed liquor under aeration and the recirculated sludgeso that there will be a substantially uniform MLSS concentration, BODand COD in the aeration tank.

While the complete mixing activated sludge system gives a substantiallyconstant microorganism population and food supply throughout theaeration tank, it is necessary that an adequate and economical supply ofoxygen also be provided within the mixed liquor to insure the propermicroorganism growth and activity.

SUMMARY OF THE INVENTION Therefore, it is an object of the presentinvention to provide an improved activated sludge process and systemhaving good oxygen transfer rate and efficiency and one which isparticularly suited for use in a complete mixing activated sludgeprocess and system.

According to the present invention, in an activated sludge sewagetreatment process and system a closed aeration chamber is provided intowhich an oxygen rich gas is injected under pressure. As pointed outheretofore, the efficiency of oxygen transfer is greater with aerateddroplets than with bubbles of gas which are diffused through the mixedliquor. Accordingly, the improved transfer also includes the generationof droplets in the enclosed area or region into which the oxygen richgas is injected. The gas is advantageously injected under a pressuregreater than atmospheric and the gas in that upper region is alsomaintained at a pressure greater than atmospheric. It is believed thatthe combination of an oxygen rich gas and the increased pressureincreases the efficiency of the oxygen transfer to the droplets ofliquor generated in the enclosure.

BRIEF DESCRIPTION OF THE DRAWING The FIGURE is a schematic anddiagramatic sectional side view showing an activated sludge system inaccordance with the present invention.

DESCRIPTION OF THE PRESENT INVENTION Referring to the drawing anaeration chamber 10 is shown which is connected to a primarysedimentation tank 12 and to a secondary sedimentation tank 14 toprovide a basic activated sludge treatment system.

The operation of the activated sludge system is well known to thoseskilled in the art. The waste to be treated isfirst placed in theprimary sedimentation tank 12 and maintained there for a sufficientdetention time to permit settleable solids to settle and the effluent isthen passed to the aeration chamber 10 where it is aerated and mixedwith the liquor in the chamber. The waste to be treated is also mixed inthe chamber with sludge which is recirculated from the secondarysedimentation tank 14. It is the sludge from the secondary tank whichprovides the seed microorganisms to build up the activated sludge in theaeration chamber 10.

In the illustrated embodiment, the aeration chamber 10 has side walls16, a bottom wall 18 and a top wall 20. Sealing means 21 are providedbetween the top and side walls inorder to seal chamber 10 and therebymaintain the above-described superatmospheric pressure in the enclosedchamber. For illustration purposes, the aeration chamber 10 may beconsidered to be square although it is to be understood that it may beof any other suitable configuration such as circular or rectangular.

The mixed liquor 22 in the aeration chamber includes the influent waste24 from the primary tank 12 and sludge 26 which has been recirculatedfrom the secondary tank 14. The waste 24 is substantially free ofsettleable solids which have been removed in the primary tank 12. Thesesettleable wastes may be removed from the tank 12 by any suitable meanssuch as a bottom outlet 28.

The influent 24 flows through a waste outlet 30 in tank 12 to an inlet32 in the aeration chamber 10. In the secondary tank 14, which containsthe aerated and settled sludge 26, is an outlet 34 which is connected toa recirculation line 36. The recirculation line 36 is connected to theaeration chamber and a dispersion outlet 37 in the chamber.

The means and method of returning the sludge and the mixing of it withthe liquor in the tank and the added waste are well known to thoseskilled in the art.

Above the upper level 38 of the liquor 22 is a gas space forming theupper region 40 of the aeration chamber 10. In the illustratedembodiment mounted near the liquid level 38 is an apparatus 42 forgenerating a spray or cloud of droplets 41 of the mixed liquor in theupper region 40. As mentioned heretofore, the transfer of oxygen todroplets id more rapid than the transfer of oxygen from bubbles of airin the mixed liquor.

The droplet apparatus may be driven by any suitable means such as amotor 44 which may be mounted on the outer surface of the top wall 20,and preferably includes a draft tube 45 for promoting the recirculationpattern of the mixed liquor as illustrated by arrows A and B.

In order to provide oxygen for the transfer to the droplets a supplytank 46 of oxygen rich gas is maintained and connected by a suitableinlet 48 and valve 47 to the upper region 40. The gas should have asufficient oxygen content to maintain at least an atmosphere of morethan 50 percent oxygen and preferably at least 80 percent so that amajor portion of the atmosphere in region 40 is oxygen.

Since the upper region 40 is completely enclosed, the oxygen rich gasentering through the inlet 48 cannot escape to the atmosphere, exceptthrough vent 50 as hereinafter described, and is, therefore, continuallyavailable to provide a rich oxygen atmosphere for the droplets which aregenerated from the liquor. As a result the liquor is constantly beingfurnished with an abundant oxygen supply. Preferably the pressure in theupper region 40 is in excess of atmospheric pressure and desirably inthe range of greater than one atmosphere to about ten atmospheres. Also,the pressure of the entering oxygen rich gas is of the same order ofpressure. If desired a pressure gauge 49 may be provided to monitor thepressure in region 40. Additional aeration gas may also be suppliedthrough a conventional air-type diffuser 54 which may be connected toany source of aeration gas such as, for example, supply tank 46.

In the aeration of the sludge during the period it is detained in thechamber 10, some gases will be generated which are preferably withdrawnfrom the chamber. To removesuch gases a vent 50 including a valve 51 isprovided and periodically the gaseous atmosphere should be vented.

The use of a droplet generator 42 in the oxygen rich atmosphere of theouter region 40 provides a high rate of oxygen transfer in the activatedsludge system, thus giving improved efficiency to the production ofmicroorganisms and, hence, reducing the retention time required forsludge in the aeration chamber.

In addition to venting the gas from the upper region through the vent 50as required, an absorber 52 is advantageously arranged in parallel withthe aeration chamber 10 and connected by means of suitable piping nothat undesirable gases, for example, carbon dioxide, may be removed asthe gas is recirculated.

Air normally contains about 21 percent oxygen by volume. Accordingly, inan aeration system using droplets, where air is used as the source ofoxygen, the partial pressure exerted by the oxygen on the droplets isapproximately 21 percent of the total pressure. However, if an oxygenrich atmosphere is created, for example one containing 84 percentoxygen, the partial pressure exerted by the oxygen will be 84 percent.Inasmuch as the driving force exerted by the oxygen is a factor in thetransfer of oxygen to the droplets of liquor, droplets exposed to an 84percent oxygen atmosphere will have approximately four times the oxygenwhen saturated compared to droplets in an air atmosphere.

The oxygen content of the gas will vary depending upon the amount ofcarbon dioxide or other gases generated in the aeration tank and thequality of the gas supplied. Commercially available oxygen gas isnormally at least 99 percent pure and in some cases it may be desirableto use a gas supply of this content. In other cases a lesser oxygencontent will be acceptable. In all cases a major portion of the gasshould be oxygen and preferably of the order of at least 80 percent.

In most cases venting of a small volume of gas and the utilization of anabsorber to remove carbon dioxide from recirculated gas from the upperregion will be satisfactory for maintaining an 80 percent oxygen level.

To show the improved efiect of an oxygen rich at mo sphere in apressurized system, a pilot plant was established. The pilot plantconsisted of an enclosed aeration chamber into which was injectedsubstantially pure oxygen under a pressure in excess of one atmosphere.A second enclosed aeration chamber was established with air injectedinto the closed chamber also under a similar pressure in excess of oneatmosphere. As a control unit a third aeration chamber was providedwhich was open to the air and it relied solely upon ambient air for itssupply of oxygen. Into each chamber one liter per day of settleddomestic sewage was fed for two hour aeration periods. The sludgerecirculation rate was set at a percent return rate. The air flow andthe oxygen flow of enclosed chambers l and 2 were set to give a steadygas flow at about two inches of water pressure.

In order to give an indication of the effect of oxygen and pressure onthe development of a sludge, all units were initially started with onlybacterial growth. The oxygen unit was the first to show the presence ofanimal growth in the form of protozoa. The air pressure unit requiredalmost three weeks to show any protozoa or growth. At the end of threeweeks the open air unit did not show any protozoa growth.

After a good sludge had been developed and placed in all three unitstests were run to determine the results of the oxygen pressure chamber,the air under pressure chamber, and the open air control chamber. Anaeration time of two hours was used and the results are set forth inTable I.

Key To Table I SF-Sewage Flow-liters/day AT-Aeration Time-hours TOTBOD-Total BOD-Milligrams/liter SOL BOD-Soluble BOD-Milligrams/liter TOTCOD-Total COD-Milligrams/liter SOL COD-Soluble CODMilligrams/liter TOTSS-Total Suspended Solids-Milligrams/liter FIX SS-Fixed SuspendedSolids-Milligrams/liter VOL SS-Volatile SuspendedSolids-Milligrams/liter As can be seen from the results of Table I theutilization of a pressure system above atmospheric resulted in improvedactivity and particularly in regard to the BOD and COD. The oxygen unit(I) showed an extremely good BOD improvement over the open air unit (3)and the air under pressure unit (2). The improved efficiency of theoxygen unit is the result of the availability of an oxygen richatmosphere, for the reasons as set forth hereinbefore.

What is claimed is:

1. An activated sludge aeration system including:

a. means forming an enclosed aeration chamber,

b. inlet means introducing oxygen-rich aeration gas comprising more than50 percent oxygen into said enclosed aeration chamber,

c. inlet means introducing biologically degradable liquid waste andrecycled sludge into said enclosed aeration chamber to form a mixedliquor therein,

d. aerator means recirculating and aerating said mixed liquor in contactwith said oxygen-rich aeration gas whereby oxygen is dissolved into saidmixed liquor and at least one other gas is given off by said mixedliquor thereby diluting the oxygen content of the oxygen-rich aerationgas above the mixed liquor,

e. vent means connected to said enclosed aeration chamber for venting aportion of the oxygen-rich aeration gas diluted with said other gas fromsaid enclosed aeration chamber to atmosphere,

. recycle conduit means having an inlet end in fluid communication withthe upper portion of said enclosed chamber at a first location and adischarge end in fluid communication with the upper portion of saidenclosed chamber at a second location for recycling some of said dilutedoxygen-rich aeration gas from a first upper portion of said enclosedaeration chamber to a second upper portion of said enclosed chamber,

adsorber means connected to said recycle conduit means intermediate saidfirst and second upper chamber portions for removing a substantialamount of said other gas from the recycled oxygen-rich aeration gas andsubstantially increasing the oxygen content of the recycled oxygen-richaeration gas returned to said enclosed aeration chamber, and

h. valve means for controlling the proportion of diluted aeration gaswhich is vented to atmosphere versus that which is recycled through saidadsorber.

2. The activated sludge system as claimed in claim 1 wherein said othergas is carbon dioxide at least a substantial portion of which is removedby said adsorber means whereby the oxygen content of the aeration gasreturned to said enclosed chamber is substantially greater than that ofthe vented gas.

3. The activated sludge aeration system as claimed in claim 1 whereinsaid inlet end of said recycle conduit TABLE I Mixed liquor InfluentPercent Temp., SS VSS H "0. w

Percent Temp., TOT FIX VOL SS VSS H C. SS SS TOT SOL TOT SOL TOT FIX VOLUnlt SF AT BOD BOD COD COD SS means is connected to said atmosphericvent means, and said discharge end is connected to said oxygen-richaeration gas inlet means, and wherein said valve means are positioned insaid vent means for controlling the amount of diluted oxygen-richaeration gas which is vented to atmosphere versus that recycled throughsaid adsorber.

4. An activated sludge aeration system including a completely enclosedaeration chamber comprising:

a. a structure having bottom, side and top walls, b. seal means betweensaid walls for maintaining a superatmospheric pressure in said enclosedaeration chamber,

inlet means introducing biologically degradable waste liquid andrecirculated sludge into said enclosed aeration chamber,

. means for completely mixing said waste liquid and said recirculatedsludge to form a mixed liquor having a substantially uniform MLSSconcentration,

. a source of oxygen-rich aeration gas containing at least 50 percentoxygen under a superatmospheric pressure greater than one atmosphere,

. conduit means connected between said source of superatmosphericoxygen-rich aeration gas and said enclosed aeration chamber throughwhich said superatmospheric oxygen-rich aeration gas is supplied to saidenclosed aeration chamber,

. valve means in said conduit means maintaining the outlet means forremoving oxygenated mixed liquor from said aeration chamber atsubstantially uniform MLSS concentration. An activated sludge aerationchamber for rapidly oxygenating biologically degradable waste liquidwith a substantially uniform MLSS concentration throughout said aerationchamber including:

a. bottom, side and top walls forming an enclosed aeration chamber,

inlet means introducing biologically degradable waste liquid andactivated sludge into said enclosed chamber to form a mixed liquor,

a source of oxygen-rich aeration gas comprising more than 50 percentoxygen under superatmospheric pressure,

. conduit means connected between said source of superatmosphericoxygen-rich aeration gas and said enclosed aeration chamber throughwhich said superatmospheric oxygen-rich aeration gas is supplied to saidenclosed aeration chamber,

. valve means in said conduit means maintaining the flowrate of saidsuperatmospheric oxygen-rich aeration gas at a rate sufficient tomaintain the pressure of said oxygen-rich aeration gas in said enclosedchamber above the level of said mixed liquor at a superatmosphericpressure but less than 10 atmospheres,

. bladed surface aerator means positioned at the surface of said mixedliquor for recirculating said mixed liquor and projecting portionsthereof into oxygen transfer contact with said superatmosphericoxygen-rich aeration gas above the level of said mixed liquor,

. second conduit means connected to said source of superatmosphericoxygen-rich aeration gas and extending to the lower portion of saidenclosed chamber substantially below the level of said mixed liquor, and

h. a submerged diffuser connected to said second conduit means andpositioned in the bottom portion of said chamber in the regionvertically below said surface aerator means for oxygenating said mixedliquor with superatmospheric oxygen-rich aeration gas in the lowerliquid portion of said chamber and producing upward flow of oxygenatedmixed liquor toward said surface aerator means whereby said surfaceaerator means and said submerged diffuser combine to produce rapidrecirculation of the mixed liquor and substantially uniform MLSSconcentration of said mixed liquor throughout said chamber.

2. The activated sludge system as claimed in claim 1 wherein said othergas is carbon dioxide at least a substantial portion of which is removedby said adsorber means whereby the oxygen content of the aeration gasreturned to said enclosed chamber is substantially greater than that ofthe vented gas.
 3. The activated sludge aeration system as claimed inclaim 1 wherein said inLet end of said recycle conduit means isconnected to said atmospheric vent means, and said discharge end isconnected to said oxygen-rich aeration gas inlet means, and wherein saidvalve means are positioned in said vent means for controlling the amountof diluted oxygen-rich aeration gas which is vented to atmosphere versusthat recycled through said adsorber.
 4. An activated sludge aerationsystem including a completely enclosed aeration chamber comprising: a. astructure having bottom, side and top walls, b. seal means between saidwalls for maintaining a superatmospheric pressure in said enclosedaeration chamber, c. inlet means introducing biologically degradablewaste liquid and recirculated sludge into said enclosed aerationchamber, d. means for completely mixing said waste liquid and saidrecirculated sludge to form a mixed liquor having a substantiallyuniform MLSS concentration, e. a source of oxygen-rich aeration gascontaining at least 50 percent oxygen under a superatmospheric pressuregreater than one atmosphere, f. conduit means connected between saidsource of superatmospheric oxygen-rich aeration gas and said enclosedaeration chamber through which said superatmospheric oxygen-richaeration gas is supplied to said enclosed aeration chamber, g. valvemeans in said conduit means maintaining the flowrate of saidsuperatmospheric oxygen-rich aeration gas at a rate sufficient tomaintain the pressure of said oxygen-rich aeration gas in said enclosedchamber above the level of said mixed liquor at a superatmosphericpressure but less than 10 atmospheres, h. said means for completelymixing said waste liquid and said recirculated sludge including bladedsurface aerator means projecting a spray of droplets of said mixedliquor into the upper region of said enclosed chamber in contact withsaid superatmospheric oxygen-rich aeration gas for transferring oxygento said mixed liquor and, i. outlet means for removing oxygenated mixedliquor from said aeration chamber at substantially uniform MLSSconcentration.
 5. An activated sludge aeration chamber for rapidlyoxygenating biologically degradable waste liquid with a substantiallyuniform MLSS concentration throughout said aeration chamber including:a. bottom, side and top walls forming an enclosed aeration chamber, b.inlet means introducing biologically degradable waste liquid andactivated sludge into said enclosed chamber to form a mixed liquor, c. asource of oxygen-rich aeration gas comprising more than 50 percentoxygen under superatmospheric pressure, d. conduit means connectedbetween said source of superatmospheric oxygen-rich aeration gas andsaid enclosed aeration chamber through which said superatmosphericoxygen-rich aeration gas is supplied to said enclosed aeration chamber,e. valve means in said conduit means maintaining the flowrate of saidsuperatmospheric oxygen-rich aeration gas at a rate sufficient tomaintain the pressure of said oxygen-rich aeration gas in said enclosedchamber above the level of said mixed liquor at a superatmosphericpressure but less than 10 atmospheres, f. bladed surface aerator meanspositioned at the surface of said mixed liquor for recirculating saidmixed liquor and projecting portions thereof into oxygen transfercontact with said superatmospheric oxygen-rich aeration gas above thelevel of said mixed liquor, g. second conduit means connected to saidsource of superatmospheric oxygen-rich aeration gas and extending to thelower portion of said enclosed chamber substantially below the level ofsaid mixed liquor, and h. a submerged diffuser connected to said secondconduit means and positioned in the bottom portion of said chamber inthe region vertically below said surface aerator means for oxygenatingsaid mixed liquor with superatmospheric oxygen-rich aeration gas in thelower liquid portion of said chamber and producing upward flow ofoxygenated mixed liquor tOward said surface aerator means whereby saidsurface aerator means and said submerged diffuser combine to producerapid recirculation of the mixed liquor and substantially uniform MLSSconcentration of said mixed liquor throughout said chamber.